There are many applications in which the accurate measurement of a time interval is useful. For example, accurate time interval measurement is used in phase detectors of digital phase-locked-loops (PLLs), in analog-to-digital converters and in high resolution measurement equipment. An industry-wide trend to replace more analog, mixed signal, and radio frequency functionality with increasingly faster digital solutions further enhances the need for high-resolution time measurement.
Time-to-digital converters (TDCs) are often used for such accurate time measurements. For example, a TDC can be used to measure the duration of time that has elapsed between a START and a STOP pulse or any other timing event. Typically, TDC circuits digitize time by utilizing the propagation delay of delay cells, which can vary significantly due to process, voltage and temperature (PVT) variations. It is generally desirable to provide a TDC that overcomes the effects of PVT variations and that can be designed and implemented independent of process parameters.